Since the invention of transistors, digital control circuits have revolutionized not only the electronics industry, but all walks of life which use or depend upon an electronic switching function for control and/or operation of an electrical circuit. The transistor, in digital applications, basically operates as an "on/off" switch. This is in contrast to analogue circuits which operate on the basic principles of measuring and multiplying the actual real-time value of electrical signals.
The present invention applies to digital, as opposed to analogue circuitry. Such digital circuits typically operate in response to series or combinations of "binary" or "logical" signal levels. While the digital signals may assume a plurality of different levels or operative states, for simplicity, most digital systems operate on a simple "two-level" or "binary" logical system. Simply stated, the digital signal assumes either a "logical one" or a "logical zero" state or level.
The actual pulse train used to implement the desired logical signal can be configured using either what is typically referred to as "positive" or "negative" logic. Positive logic refers to that condition which exists when the voltage that is used to represent a "logical one" signal is greater than that voltage level that is used to represent a "logical zero" signal. The "base" voltage (i.e. the "logical zero" voltage value) can be set at any desired voltage level, as dictated by the particular components used to implement the digital circuitry. For example, while it is common to set the base or reference level for a "logical zero" at ground or zero volts in a "positive logic" system, the base or reference voltage could be set at any other desired reference level. In contrast, "negative logic" refers to that condition which exists when the voltage level that is used to represent a "logical one" signal is less than that voltage level that is used to represent a "logical zero" signal. The characterization of a circuit as being operative in response to either "positive" or "negative" logical signals is typically referred to as the "polarity" of the circuit, and the circuit is typically referred to as operating simply on either "positive logic" or "negative logic".
Any given digital circuit necessarily operates either on a "positive" or "negative" logic basis, and requires any input signal received from outside of the circuit to be compatible with that "type" of logic on which the circuit has been designed to operate. Obviously, the application of a negative logic input signal to a positive logic digital network, would not produce the desired results from the digital network. The same would be true for application of positive logic input signals to a negative logic digital network. For this reason, electronic circuits designed to receive external input signals, are accompanied by instructions or schematic diagrams which allow the user of the circuit to apply input pulses of the proper polarity to the circuitry. It is customary for a user of the circuit to: (1) determine the circuit polarity and (2) assure that the input signals applied to such circuit are of the proper polarity, either by selecting equipment for providing input signals of the proper polarity, or by converting the input signals to the desired polarity before applying them to the circuit network.
There are many applications in the electronics industry, particularly where modular equipment is being used, wherein it is desirable to interchangeably connect a piece of electronic equipment to receive input signals from any one of a number of electronic modules, without having to take the time to determine the "polarity" of the incoming signals from the respective modules, and without having to make the decision as to whether such incoming signals are compatible with the receiving network. An example of one such situation is in the computer and word processing industries, wherein CRT consoles or monitors manufactured by one supplier may be used with control and/or drive equipment of many different manufacturers. In such an instance, it is desirable for the end user to simply. connect the CRT console to receive the output signals from the related drive modules, without having to take the time and precautions for determining the polarity compatibility of the drive module circuitry with that of the CRT console. Further, such polarity incompatibility dilemma is repeated each time the user disconnects the CRT console from one module for operation by another. As simple as it may seem, circuits have not been provided in the art for determining the polarity of the incoming signal and for "automatically" passing such signal as is, or for converting its polarity if required, to match that polarity needed by the receiving circuit.
The present invention directly addresses and solves the shortcomings of the prior art and eliminates the manual efforts involved in determining the compatibility status of an incoming signal with the requirements of the receiving circuit and for automatically making any required changes in the receiving signal to accomplish compatibility.